SubtitleFont
One font file an application hands to the subtitle typesetter, by name and bytes.
Only the typesetting engine reads these; the Kotlin dialogue tier uses the platform's own font system. The bytes are shared, not copied, and read once when a session starts typesetting, so an application may keep one instance for the life of the process.
Where fonts come from otherwise: Apple and Windows builds of libass find system fonts on their own. Android and Linux have no such provider, so those builds load a small set of the system's sans-serif faces, the fonts a container attaches, and whatever is listed here.